1. Field of the Invention
The present invention relates to an image printing apparatus, image printing method, and image processing apparatus.
2. Description of the Related Art
There is a known image printing apparatus that prints images by repeating print scans in which ink is discharged to a unit area of a printing medium while moving a print head including an array of a plurality of print discharge openings through which ink is discharged relatively in a scan direction and sub-scans in which the printing medium is conveyed in a conveyance direction substantially perpendicular to the scan direction. Such an image printing apparatus can use a so-called multipass printing method, which is a known method for forming an image by dividing data of the image to be formed using mask patterns in which print permitting pixels where printing is permitted in each scan and performing a plurality of print scans on a unit area.
It is known that the image printing apparatus using such a multipass printing method can use different mask patterns in accordance with various conditions. US Patent Publication NO. 2005/0219294 discloses setting the number of adjacent pixels in which printing is permitted in a mask pattern (hereinafter also referred to as print permitting pixels) used in printing an image on a glossy printing medium at a value larger than the number of adjacent print permitting pixels in a mask pattern used in printing an image on plain paper. According to this patent literature, the use of the above-described mask patterns can reduce the roughness of printed images and thus can print the images with suppressed decrease in glossiness.
However, when the above-described mask pattern with many locations of adjacent print permitting pixels is used, a desired image quality may not be obtained, depending on the attribute of the image.
This issue is described below in detail.
FIGS. 1A to 1E are illustrations for describing a process in printing a fine-line image as one example using mask patterns with many adjacent print permitting pixels.
FIG. 1A schematically illustrates binary data corresponding to a fine-line image. In FIG. 1A, the pixels indicated as the filled squares denote the pixels to which ink is to be discharged, and the pixels indicated as the open squares denote the pixels to which ink is not to be discharged. For character images and the like, the number of pixels to which ink is to be discharged arranged in the X direction or Y direction is smaller than that for graphical images such as pictures. Here, an example case is discussed. In this case, as one example of binary data corresponding to a fine-line image, binary data in which six pixels 700 are aligned in the X direction is processed. The pixels 700 are the ones to which ink is to be discharged. The X and Y directions are common to FIGS. 1A to 1E.
That binary data is allocated to a plurality of scans by mask patterns. Here, a case where printing is performed on a unit area by four scans is described as one example. FIGS. 1B1 to 1B4 illustrate mask patterns corresponding to first to fourth scans, respectively, on the unit area. In FIGS. 1B1 to 1B4, the pixels indicated as the filled squares denote print permitting pixels, and the pixels indicated as the open squares denote pixels where printing is not permitted (hereinafter referred to as non-print permitting pixels). In the mask patterns illustrated in FIGS. 1B1 to 1B4, the print permitting pixels are arranged such that the number of adjacent print permitting pixels is large. Specifically, each of sets 701 to 704 of print permitting pixels consists of nine print permitting pixels adjacent in the X and Y directions, and the nine print permitting pixels are arranged in a three-by-three matrix.
FIG. 1C schematically illustrates print data generated based on the binary data illustrated in FIG. 1A and the mask patterns illustrated in FIGS. 1B1 to 1B4. The pixels indicated as the filled squares denote pixels to which ink is discharged, and the digit in each of these pixels indicates what number the scan in which the ink is discharged to that pixel is. Among the six pixels to which ink is to be discharged, three pixels 705 adjacent in the X direction and corresponding to the set 701 of print permitting pixels in the mask pattern illustrated in FIG. 1B1 are the pixels to which ink is to be discharged in the first scan. Three pixels 706 adjacent in the X direction and corresponding to the set 703 of print permitting pixels in the mask pattern illustrated in FIG. 1B3 are the pixels to which ink is to be discharged in the third scan.
FIG. 1D illustrates an image printed when the first scan is performed and ink is discharged in accordance with the print data schematically illustrated in FIG. 1C. In the first scan, three ink droplets 707 are discharged to the three pixels 705 adjacent in the X direction illustrated in FIG. 1C. These ink droplets 707 are provided to locations in contact with each other on the printing medium. Thus, the ink droplets 707 are drawn to each other in directions in which they gather indicated by the arrows in FIG. 1D.
FIG. 1E illustrates an image printed when the fourth scan is completed and ink is discharged in accordance with the print data schematically illustrated in FIG. 1C. Because the three ink droplets 707 illustrated in FIG. 1D are drawn to each other, as described above, they forms a dot 708 slightly shrinking in the X direction after fixation. Similarly, three ink droplets (not illustrated) discharged to the three pixels 706 adjacent in the X direction in the third scan illustrated in FIG. 1C gather, they form a dot 709 slightly shrinking in the X direction.
When the dots are formed as described above, as is obvious in FIG. 1E, a paper blank portion 710, which is not covered with the dots, is formed in the fine-line image. Such a paper blank portion formed in the fine-line image may degrade the image quality of the fine-line image.
As described above using one example, a mask pattern with a relatively increased number of adjacent print permitting pixels is useful in improving the glossiness, but the effects on the image quality may be an issue, depending on the attribute of an image to be printed.